The present invention is related to an improved process for the removal of undesirable water from a chemical reaction for producing products in which water is deleterious to the process and/or products produced therefrom. In particular, the products are carbonate esters, and more particularly diarylcarbonates, prepared by the oxidative carbonylation of aromatic hydroxy compounds, such as by the reaction with carbon monoxide and oxygen in the presence of a catalyst generally containing a metal of group 8, 9, or 10 of the Periodic Table of Elements and a co-catalyst. The process of this invention allows continuous removal of water without depressurization of a reaction vessel.
Water removal in the oxidative carbonylation of aromatic hydroxy compounds to make diarylcarbonates is a desirable process since it enhances the productivity of the reaction and thus reduces reactor cost per unit mass of product. This is demonstrated by the improvement obtained when molecular sieves are used for water removal in reactions to make diphenyl carbonate (DPC), as described in U.S. Pat. No. 5,399,734 and in co-pending application Ser. No. 09/224,162, filed Dec. 31, 1999.
U.S. Pat. No. 5,625,091 describes water removal under reduced pressure from oxidative carbonylation reaction mixtures containing a base as a reaction component. U.S. Pat. No. 5,498,472 describes water removal from oxidative carbonylation reaction mixtures by excess process gas flow at relatively low pressures. Both of these latter two methods are specific to reaction mixtures containing a specified catalyst type which employs a quaternary salt and a base. An inert stripping agent has also been used for removing water from reaction mixtures for oxidative carbonylation of aromatic hydroxy compounds as described in U.S. Pat. No. 5,917,078.
The problem to be solved is to develop a reaction process which removes water from an oxidative carbonylation reaction mixture continuously without the requirement of using an entrained gas process or water absorbing agent. In this manner the water content inside the reactor or reactors may be kept below a prescribed value, in a manner which retains the activity of the catalyst system and minimizes the utilities requirements.
After careful study the present inventors have discovered methods for removing water of reaction in an integrated process for oxidative carbonylation of aromatic hydroxy compounds which avoids the problems of earlier methods. Thus, in one of its embodiments the present invention is a method for preparing a diaryl carbonate which comprises contacting at least one aromatic hydroxy compound with oxygen and carbon monoxide in the presence of an amount effective for carbonylation of a catalyst composition comprising the following and any reaction products thereof:
(A) at least one Group 8, 9, or 10 metal having an atomic number of at least 44 or a compound thereof;
(B) at least one guanidinium salt or onium salt; and
(C) at least one metal co-catalyst; wherein reaction water is removed by a process comprising the steps of:
(i) removing a liquid stream from an oxidative carbonylation reaction mixture in a reaction vessel;
(ii) transferring the liquid stream to a flash vessel wherein the liquid stream is subjected to reduced pressure, whereby a majority of the water is removed;
(iii) returning at least a portion of a dried liquid stream to the reaction vessel; and
(iv) optionally adding at least one of make-up aromatic hydroxy compound or other volatile constituent or catalyst component to the reaction vessel or to the dried liquid stream before return to the reaction vessel,
wherein at least a portion of diaryl carbonate is recovered from a liquid stream either before or after water removal.